Omitting your callsign would have to be the biggest threat to modern R/T. There is always the possibility that you have inadvertantly responded to a clearance given to another aircraft, or that another aircraft has inadvertantly responded to a clearance given to you. Omitting your airline designation associated with a flight number, such as "619" instead of "Flyright 619" is also a threat as there may be more than one aircraft on frequency with the same flight number.